1. Field of the Invention
The present invention relates to an intake device of an internal combustion engine.
2. Description of the Related Art
In a known engine, a rotary valve arranged in the intake port of the engine has a valve shaft eccentrically arranged relative to the axis of the intake port, to create an eccentric air flow in the intake port (Japanese Unexamined Patent Publication (Kokai) No. 58-128418).
When this engine is operating under a light load, the rotary valve is closed. At this time, an air-fuel mixture is caused to flow along one of the inner side walls of the intake port and then flow into the combustion chamber toward the circumferential direction of the combustion chamber. As a result, a strong swirl motion is created in the combustion chamber. Conversely, when the engine is operating under a heavy load, the rotary valve is rotated to the fully open position at which the valve plate of the rotary valve extends substantially in parallel to the axis of the intake port, and since the flow area of the intake port is then at its maximum, the volumetric efficiency increases.
In this engine, the valve guide for the intake valve projects downward from the upper wall of the intake port. However, where the valve guide projects into the intake port, this valve guide creates a large flow resistance to the air flow. Consequently, in this engine, a wall projecting downward from the upper wall of the intake port and surrounding the valve guide is formed on the upper wall of the intake port. This projecting wall has an air flow guide portion having a width which gradually decreases towards the upstream of the intake port. The air-fuel mixture is guided by the air flow guide portion, and thus the flow resistance is reduced.
However, in this engine, when the rotary valve is fully open, the upstream end of the air flow guide portion is not aligned with the valve plate of the rotary valve. Consequently, the air-fuel mixture stream is initially divided into two streams by the rotary valve, and then further divided into two streams by the upstream end of the air flow guide portion. As mentioned above, in this engine, when the rotary valve is fully open, since the dividing of the air-fuel mixture stream is carried out twice, the flow resistance is still large and, therefore, a problem occurs in that a satisfactorily high volumetric efficiency cannot be obtained.